Quasielastische Lichtstreuung von isoliertem Chromatin

Chromatin undergoes structural changes in dependence on the ionic strength of monovalent cations. At low ionic strength an extended chain of nucleosomes is apparent while with increasing ionic strength more compact structures are formed. Soluble chromatin was prepared from rat liver and fractionated by gel chromatography. Quasielastic light scattering experiments on chromatin were done with monovalent salt concentrations ranging from 3-150 mM. Using this method translational diffusion coefficients have been derived. These appeared to be nearly independent of monovalent salt concentrations, indicating that the hydrodynamic radius of chromatin molecules did not change. On the other hand, sedimentation coefficients were increasing according to an exponential relation. Taken together, both findings reveal a rising of the molar mass with increasing ionic strength. On the contrary, chromatin prepared in physiological salt apparently disintegrates by lowering the ionic strength. Furthermore, it could be demonstrated by earlier small angle X-ray scattering studies that the diameter of the higher order chromatin fiber remained constant at approximately 32 nm even if these structures had been reconstituted from smaller pieces starting at low ionic strength. Thus, isolated chromatin fibers are capable of self-assembling to regular quaternary structures, even if the DNA does not form a continuous strand.